Advancing miniaturization in almost all fields of microelectronics and microsystems engineering provides for a continuing development of all technologies by which the density of all type of functional units on substrates can increase. These functional units include, for example, microcontrollers, memory components, MEMS, all type of sensors or microfluid components.
Techniques for increasing the lateral density of these functional units have greatly improved in recent years. In some branches of microelectronics or microsystems engineering, the improvement is to a point that a further increase of the lateral density of the functional units is no longer possible. In microchip production, the maximum possible resolution limit for structures that are produced lithographically has been achieved. In a few years, physical or technological limitations will no longer allow any increase in the lateral density of functional units. The industry has been addressing this problem for some years by the development of 2.5D and 3D technologies. Using these technologies, it is possible to align the same or even different types of functional units to one another, stack them on top of one another, join them permanently to one another and to network them to one another by corresponding printed circuits.
One of the key technologies for the implementation of these structures is permanent bonding. Permanent bonding is defined as all methods by which substrates can be joined to one another such that they can be separated only by high energy expenditure and an associated destruction of the substrates. There are different types of permanent bonding.
One of the most important permanent bonding methods is fusion bonding, also called direct bonding or molecular bonding. Fusion bonding is defined as the process of permanently joining two substrates via the formation of covalent connections. Fusion bonds form mainly on the surface of nonmetallic-inorganic materials.
Basically, a distinction should be made between a prebond and the actual permanent bond. A prebond is defined as a connection of surfaces which forms spontaneously when two surfaces are in contact; their bonding strength is smaller than the bonding strength of the permanent bond, which is produced by a subsequent heat treatment. The bond strength caused by the prebond is. However, sufficient to transport the two substrates without causing a shift of the substrates relative to one another. Although the bond strength between the two substrates is sufficient to easily transport the substrate stack, the bond strength is so low that a repeated, nondestructive separation of the two substrates can take place with special devices. This has the major advantage that after a prebond, the structures of the two structures can be measured and their relative positions, distortions and orientations can be determined. If it is established during the measurement process that a faulty orientation and/or a local and/or global distortion of the structures is present, or there are particles in the interface, the substrate stack can be accordingly separated again and reprocessed. After a successful and verified prebond, a permanent bond is produced by heat treatment processes. During the heat treatment process, a chemical and/or physical strengthening of the connection of the surfaces of the two substrates occurs by the supply of thermal energy. This permanent bond is irreversible in the sense that a nondestructive separation of the two substrates is no longer possible. Subsequently, it can no longer be explicitly distinguished between prebond and permanent bond, but in general there is only a bond.
The most common fusion bonds are carried out on silicon and silicon oxide substrates. Silicon is used due to its semiconductor properties as a base material for the production of microelectronic components such as microchips and memories. A so-called direct bond can also form between highly polished metal surfaces. The underlying bonding properties differ from those of a fusion bond, the mechanism with which the two surfaces can make contact with one another by an advancing bonding wave, but can also be described by the same physics. The joining of two hybrid surfaces by a so-called hybrid bond would also be conceivable. A hybrid surface is defined as a surface consisting of at least two different materials. One of the two materials is generally limited to a small space while the second material surrounds the first material. For example, metal contacts are surrounded by dielectrics. When a hybrid bond is produced by the bonding of two hybrid surfaces, the bonding wave is driven mainly by the fusion bond between the dielectrics, while the metal contacts automatically meet by the bonding wave. Examples of dielectrics and low-k materials are                non-silicon based                    polymers                            polyimides                aromatic polymers                parylenes                PTFE                                    amorphous carbon                        silicon based                    silicate based            TEOS (tetraethyl orthosilicate)            SiOF            SiOCH            Glasses (borosilicate glasses, aluminosilicate glasses, lead silicate glasses, alkali silicate glasses, etc.)                        general                    Si3N4             SiC            SiO2             SiCN                        Silsesquioxanes                    HSSQ            MSSQ                        
One of the greatest technical problems in permanent joining of two substrates is the alignment accuracy of the functional units between the individual substrates. Although the substrates can be very precisely aligned to one another by alignment systems, during the bonding process itself, distortions of the substrates can occur. Due to the distortions which arise in this way, the functional units will not necessarily be correctly aligned to one another at all positions. The alignment accuracy at a certain point on the substrate can be the result of a distortion, a scaling error, a lens fault (magnification or reduction error), etc.
In the semiconductor industry, all subtopics which relate to these problems are subsumed under the term “overlay.” A corresponding introduction to this topic can be found, for example, in: Mack, Chris. Fundamental Principles of Optical Lithography—The Science of Microfabrication. WILEY, 2007, Reprint 2012.
Each functional unit is designed in a computer before the actual production process. For example, printed circuits, microchips, MEMS, or any other structure which can be produced using microsystems technology, are designed in CAD (computer-aided design). During the production of the functional units, however, it is shown that there is always a deviation between the ideal functional units, which have been engineered on a computer, and the real ones, which have been produced in a clean space. The differences can be attributed mainly to limitations of hardware, i.e., engineering problems, but very often to physical limits. Thus, the resolution accuracy of a structure which is produced by a photolithographic process is limited by the size of the apertures of the photomask and the wavelength of the light used. Mask distortions are transferred directly into the resist. Linear motors of machines can only approach reproducible positions within a given tolerance, etc. Therefore, it is no wonder that the functional units of a substrate cannot be exactly equal to computer-engineered structures. All substrates, therefore, already have a not negligible deviation from the ideal state before the bonding process.
If the positions and/or forms of two opposite functional units of two substrates are compared, under the assumption that neither of the two substrates is distorted by a joining process, it has been found that generally there is imperfect congruence of the functional units since they deviate from the ideal computer model by the above-described faults. The most frequent faults are shown in FIG. 8 (copied from: http://commons.wikimedia.org/wiki/File: Overlay—typical model terms DE.svg 24.05.2013 and Mack, Chris. Fundamental Principles of Optical Lithography—The Science of Microfabrication. Chichester: WILEY, p. 312, 2007, reprint 2012). According to the figures, it can be roughly distinguished between global and local and symmetrical and asymmetrical overlay faults. A global overlay fault is homogenous, therefore, independent of site. It produces the same deviation between two opposing functional units regardless of the position. The classic global overlay faults are faults I and II, which form by a translation or rotation of the two substrates to one another. The translation or rotation of the two substrates produces a corresponding translational or rotational fault for all functional units which are opposite at the time on the substrates. A local overlay fault arises depending on the location, mainly by problems of elasticity and/or plasticity, in this case caused mainly by the continuously propagating bonding wave. Of the described overlay faults, mainly faults III and IV are call run-out faults. These faults arise mainly by a distortion of at least one substrate during a bonding process. The functional units of the first substrate with reference to the functional units of the second substrate are also distorted by the distortion of at least one substrate. Faults I and II can, however, also arise by a bonding process, but are generally so dramatically overlain by faults III and IV that they can only be recognized and measured with difficulty.
In the prior art, there is already a system by which local distortions can be at least partially reduced. It is a matter of local distortion due to use of active control elements (see WO2012/083978A1).
In the prior art, there are initial approaches to the correction of run-out faults. US20120077329A1 describes a method for obtaining a desired alignment accuracy between functional units of two substrates during and after bonding by the lower substrate not being fixed. In this way, the lower substrate is not subjected to boundary conditions and can bond freely to the upper substrate during the bonding process. An important feature in the prior art is primarily the flat fixing of a substrate, generally by means of a vacuum device.
The run-out faults which arise are more dramatic in most cases radially symmetrically around the contact site and, therefore, increase from the contact site to the periphery. In most cases, it is a linearly increasing intensification of the run-out faults. Under special conditions, the run-out faults can also increase nonlinearly.
Under especially optimum conditions, the run-out faults can be determined not only by corresponding measuring devices (see EP2463892), but can also be described by mathematical functions. Since the run-out faults constitute translations and/or rotations and/or scalings between well-defined points, they are preferably described by vector functions. Generally, this vector function is a function f:R2→R2, i.e., an imaging standard which images the two-dimensional definition region of the local coordinates onto the two-dimensional value range of run-out vectors. Although an exact mathematical analysis of the corresponding vector fields could not be done, assumptions are made with respect to the function properties. The vector functions are with great probability cn n>=1 functions, therefore, they are continuously differentiable at least once. Since the run-out faults increase from the contact-making point toward the edge, the divergence of the vector function will probably be different from zero. The vector field is therefore with great probability a source field.
An advantage of this invention is a device and a method for bonding of two substrates with which the bond precision, especially on the edge of the substrates, is increased.
This advantage is achieved with the features of Claims 1 and 9. Advantageous developments of the invention are given in the dependent claims. All combinations of at least two of the features given in the specification, the claims and/or in the figures fall within the scope of the invention. For given values ranges, values which lie within the indicated limits should be considered disclosed as boundary values and able to be claimed in any combination.